This invention relates to palladium catalysts useful in the production of hydroxyethers which are useful as oil stain removers, water-soluble organic solvents, polar oils, emulsifiers, lubricants, humectants or the like, or as intermediates for producing surf actants, and also to a process for producing hydroxyethers by use of the palladium catalysts.
Known as hydrogenolytic processes for acetal compounds are (1) a process which uses a mixture of an alkylaluminum halide compound or an aluminum halide and lithium aluminum hydride [E. L. ELIEL et al., J. Am. Chem. Soc., 84,2371 (1962)] and (2) a process which uses a hydrogenation catalyst comprising a metal such as palladium, platinum or rhodium supported thereon in a hydrogen atmosphere (JP-A-54-135714).
However, the process (1) involves a problem in safety because the reagents to be used therein have considerable inflammability and are required in stoichiometric amounts. Thus the process is also accompanied by a problem in that a large amounts of wastes are produced. The process (2), on the other hand, is free of such problems and is advantageous. However, in the case where the acetal compound is an acetal of a polyhydric alcohol such as a cyclic acetal, the process involves a problem in that the resulting product inevitably has a mixed composition of monoethers, diethers and the like through transacetalization (or acetal exchange) or the like.
An object of the present invention is to provide a catalyst capable of obtaining a monoether with high selectivity in the synthesis of an ether and also a process for synthesizing a monoether with high selectivity through hydrogenolysis of a cyclic acetal by using the catalyst.
The present inventors have found that the above-described problems can be solved by using a catalyst comprising palladium supported on a mesoporous aluminosilicate which is a porous carrier having a substantially uniform pore size greater than that of zeolite. The inventors have also found that a catalyst especially excellent in activity in the production of an ether can be provided by supporting palladium on a mesoporous aluminosilicate treated beforehand with ammonia or a salt thereof.
Specifically, the present invention provides a process for producing an ether, which comprises reacting a cyclic acetal and hydrogen in the presence of a palladium catalyst supported on a mesoporous aluminosilicate
The present invention also provides a palladium catalyst comprising palladium supported on a mesoporous aluminosilicate which has been treated with ammonia or a salt thereof.
The term xe2x80x9cmesoporous aluminosilicatexe2x80x9d as used herein, which is a carrier in the palladium catalyst according to the present invention, means an aluminosilicate having a uniform pore size of from 2 to 50 nm. This uniform pore size can be determined by a powder X-ray diffraction pattern, and is preferably from 2 to 10 nm, particularly from 2 to 6 nm. A monoether can be obtained with high selectivity in the present invention because transacetalization is controlled inside of such mesoporous pores. Therefore, the best results can be obtained by using a catalyst having a pore size and a palladium dispersion state which are suitable for substrates.
The palladium catalyst according to the present invention can be produced by synthesizing a mesoporous aluminosilicate having a pore size greater than that of zeolite and then having palladium supported on the mesoporous aluminosilicate.
The mesoporous aluminosilicate can be synthesized, for example, by the process disclosed in Bull. Chem. Soc. Jpn., 63, 988 (1990). From the standpoint of synthesis, however, the percentage of Al based on Si is preferably 10 wt. % or lower, and the pore size of the mesoporous aluminosilicate is preferably 2 to 10 nm, especially at 2 to 6 nm.
In the present invention, it is preferred to have palladium supported on a mesoporous aluminosilicate after treating the mesoporous aluminosilicate with ammonia or a salt thereof.
As a method for treating a mesoporous aluminosilicate with ammonia or its salt in advance, it is preferred to subject a synthesized mesoporous aluminosilicate to neutralization or ion exchange with ammonia or its salt. In this method, ammonia may be used in the formof either gas or an aqueous solution. Examples of the salt of ammonia can include ammonium salts of inorganic acids such as ammonium chloride, ammonium sulfate and ammonium carbonate; and ammonium salts of lower organic acids (carbon number: 1 to 3) such as ammonium acetate, ammonium formate and ammonium lactate, with ammonium chloride being particularly preferred. The ammonia salt for use in this treatment can be used in the form of an aqueous solution. It is necessary to use ammonia or its salt in an amount sufficiently greater than that of contaminated sodium ions. Specifically, a mesoporous aluminosilicate may be dispersed at 0 to 100xc2x0 C., preferably at room temperature to 80xc2x0 C. in an aqueous solution of ammonia or its salt in a sufficiently excess amount such as 1 to 100 molar times, preferably 1 to 50 molar times as much as the molar amount of aluminum in the mesoporous aluminosilicate to bring the mesoporous aluminosilicate into contact with the solution of ammonia or its salt, followed by rinse. Before palladium is supported, the thus-treated mesoporous aluminosilicate may be calcined at 200 to 700xc2x0 C., preferably at 300 to 600xc2x0 C., or may be used without calcination for supporting palladium. By treating the mesoporous aluminosilicate with ammonia or its salt as described above before palladium is supported thereon, the amount of sodium ions contaminated in the mesoporous aluminosilicate can be significantly decreased, leading to an improvement in the palladium supporting efficiency through ion exchange and also to a significant improvement in the catalytic reaction activity of the supported palladium. When employed especially in synthesizing an ether from a polyhydric alcohol and a carbonyl compound, the reaction activity and selectivity to the monoether formation are improved significantly.
Processes for supporting palladium on the mesoporous aluminosilicate include, for example, impregnation process, ion exchange process, and CVD process. In general, impregnation process is used widely. As a process for having palladium supported on the mesoporous aluminosilicate, however, ion exchange process is preferred.
The process in which palladium is supported by ion exchange is suitable for the catalyst preparation process according to the present invention, because this process makes it possible to have palladium supported rather readily in a highly dispersed state.
Examples of palladium salts usable in this supporting process include PdCl2, Pd(OAc)2, Pd(NH4)Cl2, and [Pd(NH3)4]Cl2, with PdCl2 and Pd(OAc)2 being particularly preferred. An ion exchange process using PdCl2, Pd(OAc)2 or the like includes, for example, a process in which PdCl2 is dissolved in aqueous ammonia or Pd(OAc)2 is dissolved in a liquid mixture of acetone and water, and a mesoporous aluminosilicate or a salt thereof is dispersed in the resultant solution to bring the mesoporous aluminosilicate or its salt into contact with the solution. A process using a solution of PdCl2 in aqueous ammonia is preferred. No particular limitation is imposed on the amount of PdCl2 or the treatment temperature, but the amount of PdCl2 is preferably from 0.01 to 500 wt. %, more preferably from 0.1 to 200 wt. % based on the amount of mesoporous silicate, and the treatment temperature is preferably from 0 to 100xc2x0 C., more preferably from room temperature to 80xc2x0 C.
The amount of palladium supported on the mesoporous aluminosilicate is preferably from 0.1 to 10 wt. % of the whole amount of the catalyst. The amount greater than this range has a higher possibility of giving an adverse effect such as sintering when palladium is supported.
Subsequent to supporting of palladium on the mesoporous aluminosilicate, it is preferred to conduct calcination at 200 to 700xc2x0 C., particularly at 300 to 600xc2x0 C.
The palladium catalyst thus prepared is used as a catalyst, for example, for producing an ether by reacting a cyclic acetal with hydrogen as illustrated by the following reaction sheme. 
wherein R1 and R2 may be the same or different and represent a hydrogen atom or a linear or branched hydrocarbon group having 1 to 18 carbon atoms, R3 to R6 may be the same or different and represent a hydrogen atom, a hydroxyl group, or a substituted or unsubstituted, linear or branched hydrocarbon group or hydroxyalkyl group having 1 to 14 carbon atoms, and n denotes a number of from 0 to 2, with a proviso that R1 and R2 do not represent hydrogen atoms at the same time, and each of R1 and R2, R3 and R4, and R3 and R5, together with the adjacent carbon atoms, may form a cyclic structure.
In the cyclic acetal used in the present invention, preferred examples of R1 and R2 in the formula (1) include hydrogen atom; alkyl groups having 1 to 12 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, hexyl, heptyl, isooctyl and undecyl; and cyclic structures formed by R1 and R2, such as cyclopentane and cyclohexane.
Preferred examples of R3 to R6 in the formula (1) include hydrogen atom; hydroxyl group; alkyl groups having 1 to 4 carbon atoms, such as methyl, ethyl and propyl, and hydroxyl-substituted groups of these alkyl groups; and cyclic structures formed by R3 and R4 or R3 and R5, such as cyclopentane and cyclohexane. No particular limitation is imposed on the substituents of these hydrocarbon groups or hydroxyalkyl groups insofar as they are functional groups not preventing hydrogenolysis. Examples of such substituents are halogen atoms and oxo group. Preferably n is 0 or 1. Examples of the most preferred combination of R3 to R6 and n include R3xe2x95x90H, R5xe2x95x90hydroxymethyl, and n=0; and R3xe2x95x90R5xe2x95x90H, n=1, R4xe2x95x90H, and R6xe2x95x90OH.
The cyclic acetal (1) can be synthesized by acetalization (acetal formation) reaction from the corresponding carbonyl compound and polyhydric alcohol according to a conventional method.
Examples of the carbonyl compound are preferably linear, branched or cyclic compounds having 2 to 37 carbon atoms and containing one carbonyl group; more preferably aliphatic aldehydes having 2 to 19 carbon atoms and containing one carbonyl group, linear or branched ketones having 3 to 37 carbon atoms, and cyclic ketones having 5 to 8 carbon atoms; still more preferably aliphatic aldehydes having 2 to 12 carbon atoms and containing one carbonyl group, linear or branched ketones having 3 to 10 carbon atoms, and cyclic ketones having 5 to 6 carbon atoms. Of these, particularly preferred are propylaldehyde, butylaldehyde, isobutylaldehyde, valeric aldehyde, isovaleric aldehyde, hexylaldehyde, heptylaldehyde, octylaldehyde, isononylaldehyde, dodecylaldehyde, acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. In the present invention, one of more of these carbonyl compounds can be used.
Illustratives of the polyhydric alcohol are ethylene glycol, 1,2- or 1,3-propanediol, glycerin, 1,2-, 1,3- or 2,3-butanediol, 1,2- or 2,4-pentanediol, trimethylolethane, trimethylolpropane, xylitol, arabitol, 1,2- or 1,3-cyclopentanediol, 1,2- or 1,3-cyclohexanediol, and 2,3-norbornanediol, and are preferably glycerin and ethylene, particularly glycerin.
The ratio (molar ratio) of the polyhydric alcohol to the carbonyl compound used in the acetalization reaction ranges from 1.5 to 0.2, especially from 1.2 to 0.6. This reaction can be conducted using as an acid catalyst paratoluenesulfonic acid, methanesulfonic acid, sulfuric acid or the like in an amount of from 0.01 to 5 mol %, preferably from 0.1 to 1 mol % based on the carbonyl compound.
The above-described acetalization reaction is conducted preferably in the absence of solvent, or in an inert solvent such as xylene, toluene, benzene, octane, isooctane, heptane, hexane, cyclohexane, pentane, butane, ligroin or petroleumether, or in a mixed solvent thereof, while removing water produced in the process. Further, the acetalization reaction can be conducted under a nitrogen gas stream, in a nitrogen atmosphere or in an air atmosphere. The reaction temperature varies depending upon the boiling point of the carbonyl compound to be used, but from the viewpoint of sufficient reaction velocity and suppressing coloration and side reactions, the reaction temperature is preferably from 20 to 130xc2x0 C., particularly from 50 to 100xc2x0 C. The reaction time varies depending upon various conditions, but in general, is preferably from 1 to 30 hours. After the cyclic acetal thus obtained is neutralized and subjected to pretreatment such as filtration and washing, it can be purified by procedure such as clay treatment, crystallization or distillation.
In the hydrogenolytic reaction of the cyclic acetal obtained as described above, the present invention uses as a catalyst the palladium catalyst supported on the mesoporous aluminosilicate.
The amount of the palladium catalyst to be used in the present invention varies depending on the amount of supported palladium. For example, in the case of a palladium catalyst comprising 5 wt. % of palladium, the palladium catalyst may be used in an amount of from 0.1 to 10 wt. % based on the hydroxy compound to be used.
When reacting the cyclic acetal compound in the presence of the above-described catalyst in a hydrogen atmosphere, no particular limitation is imposed on the hydrogen pressure. The reaction may be conducted either under an elevated pressure or atmospheric pressure, but a range of from 0.1 (atmospheric pressure) to 29.4 MPa is preferred, with a range of from 0.1 (atmospheric pressure) to 19.6 MPa being particularly preferred. The reaction system can be either a closed system or a hydrogen stream system.
No particular limitation is imposed on the reaction temperature, but a range of from 10 to 220xc2x0 C., especially a range of from 30 to 200xc2x0 C. is preferred. An appropriate reaction time can be chosen depending upon the reaction temperature, the hydrogen pressure, the catalyst amount, etc., but in general, the reaction time ranges from 1 to 40 hours, preferably from 1 to 20 hours. In the reaction of the present invention, a solvent can be used. Examples of such solvents are hydrocarbon solvents such as hexane, heptane and octane. The solvent is preferably used in an amount 0.5 to 2 times by volume as much as the reaction mixture. Further, the carbonyl compound or polyhydric alcohol, the raw materials of the cyclic acetal, maybe added to the reaction system. In addition, an acidic substance such as phosphoric acid and/or a halide of a Group III element of the periodic table may also be added in trace amounts.
In this reaction, transacetalization is so controlled that the monoether can be obtained with high selectivity. Depending upon differences in the cut position of the cyclic acetal (between the 1-position and the 2-position, or between the 2-position and the 3-position), two types of monoethers can be obtained. Both of them are useful as oil stain removers, water-soluble organic solvents, polar solvents, emulsifiers, lubricants, humectants and the like and also as intermediates for producing surfactants, and can be used in these applications either after isolation or as a mixture thereof.
Purification of the monoether from the reaction mixture can be conducted by a method known per se in the art, for example, by a method such as distilling-off of the solvent, washing, recrystallization, distillation, chromatography or a combination thereof after conducting filtration to remove the catalyst.
The amounts of palladium, silicon and aluminum contained in the palladium catalyst according to the present invention can be determined by an elemental analysis such as atomic absorption spectrometry, while the amount of sodium contained in the palladium catalyst according to the present invention can be determined by X-ray electron spectrometry.